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United States Patent |
5,290,367
|
Hayashi
,   et al.
|
March 1, 1994
|
Photoelectric element
Abstract
A photoelectric element includes a first region having a light-receiving
surface. A second region having a short side length not greater than twice
the minority carrier diffusion length of the first region is provided on
at least one portion of the first region to form a photovoltaic mechanism
in conjunction with the first region. A barrier layer is provided to cover
at least those portions of the light-receiving surface of the first region
not covered by the second region, and a transparent conductive film is
provided on the barrier layer and electrically connected to at least one
second region. The voltage of the second region generated by incident
light is applied through the transparent conductive film to the
light-receiving surface of the first region to produce an electric field
in the direction inducing majority carriers.
Inventors:
|
Hayashi; Yutaka (Tsukuba, JP);
Takato; Hidetaka (Tsukuba, JP)
|
Assignee:
|
Agency of Industrial Science and Technology (Tokyo, JP);
Ministry of International Trade and Industry (Tokyo, JP)
|
Appl. No.:
|
940395 |
Filed:
|
September 3, 1992 |
Foreign Application Priority Data
Current U.S. Class: |
136/255; 136/256; 257/461; 257/E27.124 |
Intern'l Class: |
H01L 031/06 |
Field of Search: |
136/255
257/461
|
References Cited
U.S. Patent Documents
3928073 | Dec., 1975 | Besson et al. | 136/256.
|
5215599 | Jun., 1993 | Hingorani et al. | 136/255.
|
Foreign Patent Documents |
57-103371 | Jun., 1982 | JP.
| |
61-206270 | Sep., 1986 | JP | 136/255.
|
Other References
Doped Surfaces in One Sun, Point-Contactsolar Cells R. R. King, et al.
Appl. Phys. Lett., vol. 54, No. 15, Apr. 1989, pp. 1460-1462.
|
Primary Examiner: Weisstuch; Aaron
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt
Claims
We claim:
1. A photoelectric element comprising:
a first region of a semiconductor of a first conductivity type having front
and rear major surfaces, one of which constitutes a light-receiving
surface;
at least one second region provided on at least one portion of the
light-receiving side of the first region to form a photovoltaic mechanism
in conjunction with the first region, said second region having a shorter
side, in plan view, whose length is not more than twice the minority
carrier diffusion length of the first region;
a barrier layer covering at least those portions of the light-receiving
surface of the first region at which the second region is not provided;
and
a transparent conductive film in electrical contact with the at least one
second region and also in contact with the barrier layer, for enabling a
voltage generated by light incident on the light-receiving surface to be
applied to the light-receiving surface under the barrier layer as a bias
voltage of a polarity inducing majority carriers of the semiconductor
constituting the first region.
2. A photoelectric element according to claim 1, wherein:
the second region is constituted as a plurality of separate subregions or
as a continuous region having a plurality of spaced apart parts;
the distance between peripheries of adjacent ones of the plurality of
subregions or between adjacent parts being not greater than
(2LnkT/q).multidot.1n{(A.sub.1 +A.sub.2)/A.sub.2 }V.sub.0
where
L is the minority carrier diffusion length of the semiconductor
constituting the first region,
A.sub.1 is the total effective light-receiving area constituted by the
portion of the major surface of the light-receiving side of the first
region that is not covered by the second region,
A.sub.2 is the total light-receiving area of the second region,
V.sub.0 is the open-circuit voltage when A.sub.1 =0;
n is the n value in the voltage-current equation for the diode
characteristics when A.sub.1 =0
k is Boltzmann's constant,
T is absolute temperature, and
q is electronic charge.
3. A photoelectric element according to claim 1, wherein a part of the
transparent conductive film constitutes the second region.
4. A photoelectric element according to claim 1, further provided with a
third region disposed between and electrically connecting the transparent
conductive film and the second region.
5. A photoelectric element according to claim 1, further comprising a first
electrode electrically connected with the transparent conductive film, a
second electrode provided over the major surface of the first region on
the side that is opposite from the light-receiving surface thereof, and a
high-concentration impurity region provided on the opposite major surface,
for electrically connecting the second electrode with the opposite major
surface.
6. A photoelectric element according to claim 5, further comprising a
second barrier layer disposed between the opposite major surface of the
first region and the second electrode, said second barrier layer being
disposed at portions other than those where the high-concentration
impurity region and the second electrode are in contact.
7. A photoelectric element according to claim 1, further comprising a first
electrode electrically connected with the transparent conductive film, a
second electrode provided over the major surface of the first region on
the side that is opposite from the light-receiving surface thereof, and a
heterojunction forming region for repelling minority carriers provided
between the second electrode and the first region.
8. A photoelectric element according to claim 1, wherein the portions of
the major surface on the light-receiving side of the first region that are
not covered by the second region contain impurity atoms of the same
conductivity type as that of the first region and in a concentration that
does not exceed the degeneracy concentration.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a photoelectric element
2. Description of the Prior Art
Improvement of the performance characteristics of photoelectric elements
has been approached from various angles. Of particular note regarding
techniques for enhancing the conversion efficiency of such an element by
increasing its open-circuit voltage are those disclosed in Japanese Patent
Laid Open No. 57-103371 and Appl. Phys. Lett., Vol. 54, No. 15, 1989, pp.
1460-1462.
The former of these teaches improvement of the open-circuit voltage and the
fill factor of a SnO.sub.2 /Si heterojunction solar cell by a technique
related to a method for depositing the SnO.sub.2.
The latter aims at achieving an efficiency improvement by forming junctions
referred to as point contacts on the major surface of the substrate
opposite the light-receiving surface (i.e. the rear surface) What this
amounts to structurally is forming point-like P.sup.+ regions for
junction formation at a prescribed pitch on the rear surface of a
high-resistance n-type substrate and connecting the P.sup.+ regions of
each row thereof by forming a plurality of stripe-like positive electrodes
on the rear surface of the substrate. Moreover, stripe-like negative
electrodes are formed interdigitedly adjacent to the stripe-like positive
electrodes and good ohmic contacts are established between the negative
electrodes and the high-resistance n-type substrate through point-like
n.sub.+ contact regions formed at a prescribed pitch also on the rear
surface of the substrate.
The method disclosed in the aforesaid Laid-Open Patent document focuses on
improving the efficiency of the heterojunction itself and, as such, does
not take the shape and layout on the light-receiving side of the element
into consideration as factors in enhancing the open-circuit voltage and
conversion efficiency.
On the other hand, while the teaching of the Appl. Phys. Lett. article aims
at a structural improvement, the element structure that it proposes
involves problems that derive from the fact that the positive and negative
electrodes are present only on the rear surface of the substrate and that
the two types of electrodes are formed alternately by connecting the
members of each type with a plurality of stripes. For example, there is a
high risk of shorting owing to thick solder joints, the adherence of
electrically conductive foreign matter, and other such causes. The
structure also has a drawback from the materials aspect. Specifically,
since the current collection junctions are formed only on the rear surface
of the substrate opposite from the light-receiving surface, the efficiency
decreases, not increases, unless a material is used in which the carrier
diffusion length is sufficiently greater than the substrate thickness.
OBJECTS AND SUMMARY OF THE INVENTION
The present invention was completed in light of the foregoing circumstances
and has as one of its objects to provide a photoelectric element that
enables a marked improvement in photoelectric conversion to be achieved
through an increase in open-circuit voltage, without entailing the risk of
shorting between the positive and negative electrodes.
Another object of the invention is to provide a photoelectric element able
to achieve a practical level of conversion efficiency even when it is
constituted using a material exhibiting a short carrier diffusion length.
For achieving the foregoing objects, the present invention provides a
photoelectric element comprising a first region of a semiconductor of a
first conductivity type having front and rear major surfaces, one of which
constitutes a light-receiving surface; at least one second region provided
on at least one portion of the light-receiving side of the first region to
form a photovoltaic mechanism in conjunction with the first region and
having a shorter side in plan view, whose length is not more than twice
the minority carrier diffusion length of the first region; a barrier layer
covering at least those areas of the light-receiving surface of the first
region at which the second region is not provided; and a transparent
conductive film in electrical contact with the at least one second region
and also in contact with the barrier layer for enabling a voltage
generated by light incident on the light-receiving surface to be used for
applying to the light-receiving surface under the barrier layer a bias
voltage in the direction inducing carriers that are majority carries with
respect to the semiconductor constituting the first region; a first
electrode being provided on the transparent conductive film and a second
electrode being provided on the major surface of the first region on the
side opposite from the light-receiving surface thereof.
The danger of shorting is thus eliminated in the photoelectric element
according to the invention by the provision of the positive and negative
electrodes on opposite major surfaces of the first region. In addition,
since the length of the shorter side of the second region is not greater
than the minority carrier diffusion length of the first region and the
voltage generated is applied through the transparent electrode to the
light-receiving surface of the first region as a bias voltage so as to
apply an electric field in the direction causing induction of majority
carriers, the open-circuit voltage is increased and the photoelectric
conversion efficiency improved The invention therefore provides a
practical photoelectric element capable of achieving improved
photoelectric conversion efficiency even when applied to a material whose
carrier diffusion length is too short for use in the prior art
photoelectric elements.
These and other objects and features of the invention will be more apparent
from the following description made with reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(A) is a schematic cross sectional view of an embodiment of the
photoelectric element according to the invention.
FIG. 1(B) is a schematic plan view of the element of FIG. 1.
FIG. 2(A) is an explanatory view showing a stage in the fabrication of the
element of FIG. 1(A), specifically the manner in which masking layers and
high-concentration impurity regions are formed with respect to the first
region.
FIG. 2(B) is a view for explaining the formation of the second region in
the first region in the stage following that shown in FIG. 2(A).
FIG. 2(C) is a view for explaining the formation of the transparent
conductive film on the second region and a barrier layer in the stage
following that shown in FIG. 2(B).
FIG. 3(A) is an explanatory view showing an example of an energy band
diagram of the photoelectric element according to the invention.
FIG. 3(B) is an explanatory view showing the energy band diagram when the
photoelectric element is exposed to light.
FIG. 4(A) is an explanatory view showing the photoelectric element
according to the invention formed on the first region with the barrier
layer and a heterojunction forming region.
FIG. 4(B) is an explanatory view showing the first region further formed
with the second region.
FIG. 4(C) is an explanatory view showing the element completed by the
formation of the transparent conductive film, the first electrode, and the
second electrode. FIG. 5(A) is an explanatory view showing an example of
the shape of the second region of the photoelectric element according to
the invention as seen in plan view.
FIG. 5(B) is an explanatory view of another example of the shape of the
second region as seen in plan view.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a preferred embodiment of the photoelectric element according
to the invention and FIG. 2 shows the fabrication steps of the same.
As shown in FIG. 2(A) the main structural element in the photoelectric
element according to the invention is a first region 10 formed of a
semiconductor having a conductivity type (i.e. not intrinsic). This may,
for example, be obtained from an ordinary silicon wafer, a III-V group
semiconductor wafer, or a sheet or film of either of these materials.
While it may be of either conductivity type, it is of p-type in the
embodiment under discussion. Either the front or rear major surface of the
first region 10 serves as the light-receiving surface of the completed
element. In the drawings, the major surface at the top is designated as
the light-receiving surface 11 and the major surface on the opposite side
as the opposite surface 12.
A masking layer 31 is formed on the light-receiving surface 11 and a
masking layer 61 is formed on the opposite surface 12. As will be
explained later, these masking layers can also serve as barrier layers 30,
60, and to indicate this, the reference numerals 30 and 60 are shown in
parentheses following the reference numerals 31 and 61 in FIG. 2(A). The
fabrication stage shown in FIG. 2(A) also includes the formation in the
masking layer 61 formed on the opposite surface 12 of a prescribed number
of openings of prescribed shape arranged in a prescribed pattern and the
selective formation via the openings of high-concentration impurity
regions 70 of the same conductivity type as the first region 10. The
high-concentration impurity regions 70 are formed by an appropriate known
method such as thermal diffusion, ion implantation, plasma doping,
chemical vapor deposition, epitaxial growth or the like and are provided
for improving the photoelectric conversion efficiency of the element by
repelling minority carriers and for establishing good ohmic contact with a
second electrode 52 formed later. The high-concentration impurity regions
70 can be formed either by introducing an impurity into the first region
in the manner just described or by lamination onto the opposite surface
12.
Next, as shown in FIG. 2(B), a prescribed number of openings of prescribed
shape arranged in a prescribed pattern are formed in the masking layer 31
formed on the light-receiving surface 11 and the second region 20 capable
of forming a rectifying junction with the first region 10 is formed via
the openings by an appropriate known method such as thermal diffusion, ion
implantation, plasma doping, chemical vapor deposition, epitaxial growth
or the like. In the case where the first region 10 is a p-type
semiconductor as in the foregoing, an ordinary n-type semiconductor is
appropriate for use as the second region 20. Alternatively, the second
region 20 can be formed as a laminated structure comprised of a plurality
of layers. For example, it can be a laminated structure consisting of i
layers and n layers. Prior art technologies can be used for the
fabrication of the laminated structure. In some circumstances, the surface
portion forming the second region 20 on the first region 10 can be a
laminated structure.
Irrespective of the formation method used, it suffices for the second
region 20 to constitute a known photovoltaic mechanism in conjunction with
the first region. The material thereof is selected in consideration of the
conductivity type of the first region 10 from among silicides enabling
hole injection, metals enabling electron injection, and the like.
Next, in the case where the masking layers 31, 61 used in the step of FIG.
2(B) are unable to double as barrier layers, they are removed. Then, as
shown in FIG. 2(C), a barrier layer 30 is formed on the light-receiving
surface 11 and a barrier layer 60 is formed on the opposite surface 12,
whereafter contact openings are formed in the barrier layer 30 on the
light-receiving side for exposing the second region 20 and a transparent
conductive film 40 is formed to be coextensive with the whole area of the
light-receiving surface of the first region 10. In addition, on the
opposite surface 12, the masking layer 61 used in the step of FIG. 2(B)
and removed in case it is incapable of serving as a barrier layer 60 is
replaced by a freshly formed barrier layer 60 which is then formed with
contact openings for exposing the surfaces of the high-concentration
impurity regions 70 formed earlier.
The completed photoelectric element according to the first embodiment shown
in FIG. 1 is then obtained by forming a first electrode 51 on the
transparent conductive film 40 and the second electrode 52 on the barrier
layer 60. The first electrode 51 is formed on the transparent conductive
film 40 in a narrow stripe pattern or the like so as to minimize
interference with incoming light, while the second electrode 52 can be
formed to be coextensive with the entire opposite surface 12. In addition,
when the transparent conductive film 40 and the second region 20 are not
in ohmic contact (ohmic contact thereof not being indispensable), the
first electrode 51 can be formed in direct contact with the second region
20. To establish ohmic contact between the transparent conductive film 40
and the second region 20, a third conductive region 21 can be formed in
between the second region 20 and the transparent conductive film 40.
In the photoelectric converter of this arrangement, when light falling
incident on the light-receiving surface 11 produces a voltage in the
second region 20 by the known photovoltaic principle, the fact that the
transparent conductive film 40 is in electrical contact with both the
second region 20 and the upper surface of the barrier layer 30 causes a
voltage bias to be applied to the area of the light-receiving surface 11
positioned under the barrier layer 30. Since the polarity of the bias
voltage is in the direction causing induction at the surface of the
light-receiving surface 11 of what are majority carriers for the first
region 10, formation of an inversion layer is suppressed. This surface
condition is hereinafter referred to as "electrically accumulated".
As a result, this self-biasing effect increases the conversion efficiency
and the open-circuit voltage. Tests conducted by the inventors show that,
relative to a photoelectric element without this type of self-biasing
structure, the increase in the open-circuit voltage V.sub.OC amounts to
(nkT/q).multidot.1n{(A.sub.1 +A.sub.2)/A.sub.2 }
where A.sub.1 is the total effective light-receiving area constituted by
the part of the major surface on the light-receiving side of the first
region not covered by the second region, A.sub.2 is the total
light-receiving area of the second region, k is Boltzmann's constant, T is
absolute temperature, q is electronic charge, n is the n value (ideality
factor) in the voltage-current equation for the diode characteristics when
A.sub.1 =0.
However, for obtaining the aforesaid effect, it is necessary for at least
the short side length of the second region 20 as seen in plan view to be
not greater than twice the minority carrier diffusion length of the first
region 10.
The term "short side length" used for convenience here requires further
definition as it applies to cases in which the shape of the second region
is other than that of a rectangle having longer and shorter sides. Where
the shape is circular it is defined as the diameter of the circle and, for
arbitrary shapes is more generally defined as the value obtained by
erecting inwardly directed perpendicular line pairs on the periphery of
the plan-view shape of the second region and averaging the lengths of
those perpendicular pairs which intersect at equal distances from the
periphery and whose points of intersection describe the longest locus
among all line pairs intersecting at equal distances from the periphery.
This will be explained again later with respect to the drawings.
Moreover, where the second region 20 consists of a plurality of subregions,
for preventing loss of the open-circuit voltage increase effect it is
necessary that, as shown in FIG. 1(B), the distance L.sub.S between the
proximal points on the periphery of adjacent subregions of the second
region 20 be not greater than
(2LnkT/q).multidot.1n{(A.sub.1 +A.sub.2)/A.sub.2 }/V.sub.o
where L is the minority carrier diffusion length of the semiconductor
constituting the first region, and V.sub.o is the open-circuit voltage
when A.sub.1 =0.
In the example shown in FIG. 1(B), the second region 20 is constituted as a
group of mutually independent subregions that are circular in plan view
and located at the intersections of a rectangular lattice pattern. It is
further possible to provide a second region subregion 20' at the center of
each lattice mesh. In this case, the shortest distance between adjacent
subregions becomes that indicated by the symbol L.sub.S' in FIG. 1(B).
In addition, the second region 20 used in the present invention can have a
continuous plan-view pattern such that a plurality of parts thereof can be
counted within at least one section perpendicular to the major surface. An
example of such an arrangement is shown in FIG. 5(A).
More specifically, it suffices for the second region 20 to have a plan view
shape such that a plurality of parts thereof can be counted within a
prescribed section taken such as indicated by the line A--A in FIG. 5(A).
In this case, it is possible to define the length W.sub.S of the short
side of the plurality of individual parts and the distance L.sub.S between
adjacent parts.
Moreover, if the plan-view shape of the individual subregions when the
second region is constituted of a plurality of subregions is rectangular
or if the general plan-view shape at the parts where the plurality of
parts is counted is rectangular, the short side length W.sub.S is simply
that, or, in the case of a square, is the length of one side. In the case
of a circle, it is the circle diameter. (The length W.sub.S of one
circular subregion of the second region 20 is indicated in FIG. 1(B).). It
is also possible to define the short side length more generally so as to
apply to all types of plan-view patterns.
Consider, for example, the arbitrary pattern of the second region 20 shown
in FIG. 5(B). Short side length W.sub.S can be defined as the value
obtained by erecting inwardly directed perpendicular line pairs on the
periphery of the plan-view shape of the second region and averaging the
lengths of those perpendicular pairs (H.sub.S1, H.sub.S1), (H.sub.S2,
H.sub.S2), . . . (H.sub.Sm, H.sub.Sm) which intersect at equal distances
from the periphery and whose points of intersection describe the longest
locus T.sub.S, among all line pairs intersecting at equal distances from
the periphery. On the condition of taking a large enough sampling number
m, therefore, the short side length can be obtained as
W.sub.S =2 (H.sub.S1 +H.sub.S2 + . . . +H.sub.Sm)/m.
This explanation with respect to FIG. 5 also applies to the second
embodiment of the invention to be described later with respect to FIG. 4.
If at the time the photoelectric element is exposed to light a large amount
of current should flow from the transparent conductive film side to the
first region via the barrier layer 30 (i.e. if the barrier layer 30 has
low resistance), it will not be possible to obtain the self-biasing
effect. While in view of the principle involved, the barrier layer 30
should preferably be an insulator, as a practical matter it suffices for
it to be a high-resistance layer or a wide-gap semiconductor layer able to
form an adequately large potential barrier with respect to the first
region 10 or the transparent conductive film 40, and, from the structural
viewpoint, this is more convenient. In other words, it is possible to use
as the barrier layer 30 in the present invention any material layer which
passes no more than a negligible amount of current relative to the
photoelectric current density produced by the incident light.
FIG. 3 shows an example, not intended to be limitative, of the band profile
in the case where a wide-gap semiconductor is used for the barrier layer
30 and a p-type semiconductor is used for the first region 10. In this
figure, the conduction bands of the individual regions are indicated by
the reference symbol CB and the valence bands by the symbol VB. When light
impinges on the first region 10, producing a potential in the transparent
conductive film 40 through the second region 20, the state shown in FIG.
3(A) at the time of no light input changes to that shown in FIG. 3(B).
Specifically, the bias effect acting through the large potential barrier
produced by the presence of the barrier layer 30 causes band bending so
that holes, which are majority carriers, are induced in the
light-receiving surface 11 of the first region 10.
FIG. 4 shows the structure of another photoelectric element according to
the invention, along with an example of the steps for fabricating the
same.
First, as shown in FIG. 4(A), a barrier layer 30 is at this stage formed
directly on the light-receiving surface 11 of a first region 10, which may
be of p-type semiconductor, and a heterojunction forming region 71, for
example, is formed on the opposite surface 12.
Next, as shown in FIG. 4(B), openings of a prescribed shape are formed in
the barrier layer 30 in a prescribed pattern and, in entirely the same
manner as that explained regarding the earlier embodiment, a second region
20 of a material able to form a photovoltaic mechanism in conjunction with
the first region 10 is formed by filling the openings.
Then, as shown in FIG. 4(C) a transparent conductive film 40 is formed
continuously over the second region 20 and the barrier layer 30, a first
electrode 51 is formed on the transparent conductive film 40, and at the
opposite surface 12 a second electrode 52 is formed on the heterojunction
forming region 71. This completes the element.
The conditions required by the invention which were explained earlier with
respect to the first embodiment also have to be met by this second
embodiment. Specifically, the short side length of the second region 20 as
seen in plan view has to be not more than twice the minority carrier
diffusion length of the first region 10, and when the second region 20 is
constituted of a plurality of discrete subregions as shown in FIG. 4 or is
constituted such that a plurality of parts thereof can be counted at least
within one section taken perpendicular to the major surfaces, the distance
between adjacent subregions or parts has to be not greater than
(2LnkT/q).multidot.1n{(A.sub.1 +A.sub.2)/A.sub.2 }/V.sub.o
times the minority carrier diffusion length L. Insofar as the embodiment
having the structure shown in FIG. 4 satisfies these conditions, it is
able to manifest the effect of the invention as regards increased
open-circuit voltage and enhanced conversion efficiency in the same way as
the first embodiment.
In the embodiment of FIG. 4, instead of being formed with the
high-concentration impurity regions 70 of the first embodiment the
opposite surface 12 is formed over the whole thereof with the region 71
capable of forming a heterojunction with the first region 10. Like the
high-concentration impurity regions 70, the heterojunction forming region
71 also serves to repel minority carriers and, as such, contributes to
improvement of the conversion efficiency. The heterojunction forming
region 71 does not, however, have to be provided over the entire area of
the opposite surface 12. Provision over only a part thereof suffices
Therefore, as indicated by reference numeral 71 in parentheses in FIGS. 1
and 2, the high-concentration impurity regions 70 of the first embodiment
can also be replaced by heterojunction forming regions 71. Conversely, as
indicated by the reference numeral 70 in parentheses in FIG. 4, the
heterojunction forming region 71 of the second embodiment can be replaced
by a high-concentration impurity region 70 fully or partially covering the
opposite surface 12.
Similarly to what was discussed earlier with regard to the first region 10,
and again without intention of limiting the invention, it can be pointed
out that in either of the foregoing embodiments, where the first region 10
is constituted of silicon, the material of the barrier layers 30 and 60
can be selected from among silicon oxide, silicon nitride, SIPOS, gallium
phosphide and the like, and where it is constituted of a III-V group
semiconductor, and particularly where it is constituted of gallium
arsenide, the material thereof can be gallium aluminum arsenide or the
like.
As regards the heterojunction forming region 71, on the other hand, free
selection of combinations from among a fairly broad range of known
prior-art materials is possible. For example, when the first region 10 is
constituted of p-type silicon, the material of the region 71 can be
selected from among p-type hydrogenated amorphous silicon, hydrogenated
microcrystalline silicon, amorphous silicon carbide, p-type gallium
phosphide and the like.
As regards the second region 20, while the invention does not directly
specify the material or physical properties of this region, in cases where
the first region 10 is constituted of p-type silicon, it can, as was
mentioned earlier, be selected from among n-type and i-type hydrogenated
amorphous silicon.
While the transparent conductive film 40 is not itself encompassed by the
principle of the invention, it is noted that the material thereof can be
selected from among indium tin oxide, tin oxide, zinc oxide and the like.
Although the material of the first region 10 has to be taken into
consideration in selecting the material of the transparent conductive film
40, depending on the properties of the first region 10 it is in some cases
possible for the transparent conductive film 40 also to provide the
function of the second region 20. That is to say, a part of the
transparent conductive film 40 can constitute the second region 20.
When the transparent conductive film 40 and the second region 20 are formed
separately, it is possible to electrically contact the film and the region
through an additionally provided separate third region 21 (in the manner
indicated by the phantom line at the left side of FIG. 1(A), for example)
insofar as good electrical contact can be maintained therebetween.
It is preferable for the transparent conductive film 40 to be of the
so-called textured type so as to enable it to prevent reflection.
It is also preferable to enhance the passivation of the surface portions of
the light-receiving side of the first region 10 which are not covered by
the second region 20 by addition to a level not exceeding the degeneracy
concentration of impurity atoms of the same conductivity type as that of
the first region, because this produces an effect which, working
synergistically with the fundamental effect of the invention, enhances the
short wavelength sensitivity of the photoelectric element and thus further
increases its overall conversion efficiency.
According to the invention, the provision of the second region so as to
meet the condition that its short side length be not greater than twice
the minority carrier diffusion length of the first region enables
improvement of the open circuit voltage of the element. By applying the
voltage produced in the second region through the transparent conductive
film to the light-receiving surface of the first region as a bias voltage,
the light-receiving surface can be electrically accumulated and stabilized
by self-biasing, without need for any outside voltage source. As a result,
the photoelectric element enjoys an improvement in terms of both
open-circuit voltage, conversion efficiency, and stability.
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